1. Field of the Invention
The invention relates to a technology for controlling an automatic transmission of a vehicle at the time of a downshift while the vehicle is coasting in a fuel-cut state and, more particularly, to a technology for controlling a lockup clutch provided in a torque converter of the automatic transmission.
2. Description of the Related Art
While a vehicle is coasting in a fuel-cut state, a control of oil pressure of a lockup clutch is sometimes executed in order to prevent a rapid fall of the engine rotation speed. For example, Japanese Patent Application Laid-Open Publication No. 6-331023 (patent literature I) discloses a technology in which the oil pressure of the lockup clutch is controlled so that a rotation speed difference (=NT−NE, hereinafter referred to as “slip rotation speed”) between the torque converter turbine rotation speed NT and the engine rotation speed NE is kept at a target value. According to this control, the engine rotation speed is maintained by rotation of the turbine, and therefore does not rapidly fall, so that the fuel-cut state during coasting can be maintained for a long time.
With reference to FIGS. 6A to 6C, transitions of characteristic values of a vehicle equipped with a related-art lockup clutch control device will be described in conjunction with a downshift from a fourth speed ratio to a third speed ratio.
FIG. 6A indicates transitions of the turbine rotation speed NT and the engine rotation speed NE. After a 4-to-3 downshift instruction is output at a time point of T(0), the turbine rotation speed NT starts to increase at a time point T(2) due to engagement of an engaging-side clutch. After an inertia phase begins at a time point T(3), the engine rotation speed NE starts to increase. The inertia phase refers to a stage where the inertia force in an engine rotation system changes. If the feedback control of the oil pressure of the lockup clutch is not executed, the turbine rotation speed NT increases as the downshift progresses, and reaches the rotation speed of the third-speed running of the vehicle at a time point T(7) (corresponding to a dotted-line portion of NT in FIG. 6(A)).
FIG. 6B illustrates the transition of the slip rotation speed based on the feedback control by the related-art control device. This oil pressure feedback control is directed, as its control object, to a target slip rotation speed that is set at a predetermined constant rotation speed. A “calculated slip rotation speed” represents a rotation speed difference at the lockup clutch, that is, the difference between the turbine rotation speed NT of the automatic transmission and the engine rotation speed NE (NT−NE).
However, if the aforementioned feedback control is executed at the time of a downshift while the vehicle is coasting in a fuel-cut state, the following problems may occur.
Firstly, the completion of the downshift may be delayed in comparison with the case where the feedback control is not executed. Referring to FIG. 6A, if the feedback control of the lockup clutch oil pressure is executed, the turbine rotation speed NT begins to decrease at a time point T(4), and then increases and reaches the rotation speed of the 3rd-speed run at a time point T(8) (corresponding to a solid-line portion of NT in FIG. 6A). Thus, the time from the beginning of the downshift until the turbine rotation speed NT reaches a predetermined rotation speed becomes longer, that is, a delay in completion of the downshift results.
Secondly, a shock may occur with changes in torque due to a rapid rise in the lockup oil pressure or external disturbances caused as a result of the lockup clutch being temporarily engaged. More specifically, referring to FIG. 6B, the calculated slip rotation speed changes in accordance with the difference between the state of change in the turbine rotation speed NT and the state of change in the engine rotation speed NE during a time period from T(1) to T(6), and thus, the calculated slip rotation speed deviates from the target slip rotation speed. To cope with such deviations, a control for bringing the calculated slip rotation speed closer to the target slip rotation speed is executed This control may sometime cause a problem including a shock generated due to external disturbances occurring as a result of the lockup clutch being temporarily engaged or due to changes in torque as a result of the lockup oil pressure being temporarily and rapidly increased, in order to reduce the aforementioned deviation of the slip rotation speed(at a time point T(4) in FIG. 6C).
A third problem is that degradation of a clutch friction member is caused since an engaging-side clutch (a clutch engaged to establish the third speed ratio in the aforementioned case) needs to perform a work for raising the turbine rotation speed to a post-downshift turbine rotation speed. Furthermore, exacerbation of a shock due to the performance of such a work is also a problem.